This invention relates to an analytical method for determining the concentration of an inhibitor that comprises hexamethylenetetramine in inhibited acidic pickling solutions. Hexamethylenetetramine, either alone or in combination with other inhibitors, is frequently present in such pickling solutions, which are widely used to pickle ferriferous metallic objects that have acquired scales or other bulk oxide containing layers during metallurgical processing. These inhibitors are generally used at relatively low concentrations in an aqueous pickling solution that also contains at least one strong mineral acid.
When the concentration of amine inhibitors in a pickling solution is sufficiently high, the pickling solution will dissolve all of the unwanted oxide layers from the surface of a ferriferous object while dissolving little or no iron from the metallic part of the object that underlies the unwanted oxide layers. However, if the concentration of inhibitor is too low, the pickling solution will rapidly dissolve the metallic part of a ferriferous object (unless the ferriferous object is made of an alloy that spontaneously passivates in the particular pickling solution being used). Such dissolution of the underlying metal is never desirable in a pickling process. Therefore, it is important to maintain the concentration of inhibitor at a sufficient level to prevent unwanted dissolution at all times during the process.
On the other hand, amine inhibitors are considerably more expensive than the other components of a pickling solution, and some mechanical loss of pickling solution is essentially inevitable in any practical pickling process, so that it is not economical to use a large excess of the inhibitor to avoid unwanted dissolution of the metallic parts of the object being pickled. Furthermore, even aside from mechanical losses, under many circumstances the concentration of effective amine inhibitors in a pickling solution will decrease during use or even storage of the pickling solution for reasons that are not fully understood. It is therefore advantageous to have a means of analytically determining the concentration of inhibitor(s) in a used pickling solution, in order to know whether more inhibitor needs to be added, and if so how much more, in order to maintain the action of the pickling solution at or near optimum.
It is relatively easy to determine the concentration of hexamethylenetetramine in a freshly made pickling solution. However, even optimal pickling solutions dissolve some iron from the scales they are intended to remove, and it has been found that dissolved iron often interferes with conventional analytical methods for hexamethylenetetramine. It is accordingly the major object of this invention to provide a method for analytically determining the concentration of hexamethylenetetramine in an acidic pickling solution that contains hexamethylenetetramine as an inhibitor, either alone or with other inhibitor substances, and also contains dissolved iron. Other alternative, concurrent, and/or subordinate objects will become apparent from the description below.
Except in the claims and the operating examples, or where otherwise expressly indicated, all numerical quantities in this description indicating amounts of material or conditions of reaction and/or use are to be understood as modified by the word "about" in describing the broadest scope of the invention. Practice within the numerical limits stated is generally preferred, however. Also, throughout the description, unless expressly stated to the contrary: percent, "parts of", and ratio values are by weight or mass; the term "polymer" includes "oligomer", "copolymer", "terpolymer" and the like; the description of a group or class of materials as suitable or preferred for a given purpose in connection with the invention implies that mixtures of any two or more of the members of the group or class are equally suitable or preferred; description of constituents in chemical terms refers to the constituents at the time of addition to any combination specified in the description or of generation in situ within the composition by chemical reaction(s) noted in the specification between one or more newly added constituents and one or more constituents already present in the composition when the other constituents are added, and does not preclude unspecified chemical interactions among the constituents of a mixture once mixed; specification of constituents in ionic form additionally implies the presence of sufficient counterions to produce electrical neutrality for the composition as a whole and for any substance added to the composition; any counterions thus implicitly specified preferably are selected from among other constituents explicitly specified in ionic form, to the extent possible; otherwise such counterions may be freely selected, except for avoiding counterions that act adversely to an object of the invention; the word "mole" means "gram mole", and the word itself and all of its grammatical variations may be used for any chemical species defined by all of the types and numbers of atoms present in it, irrespective of whether the species is ionic, neutral, unstable, hypothetical, or in fact a stable neutral substance with well defined molecules; the terms "solution", "soluble", "homogeneous", and the like are to be understood as including not only true equilibrium solutions or homogeneity but also dispersions that show no visually detectable tendency toward phase separation over a period of observation of at least 100, or preferably at least 1000, hours during which the material is mechanically undisturbed and the temperature of the material is maintained within the range of 18-25.degree. C.; the first definition of an acronym or other abbreviation applies to all subsequent uses of the same acronym or other abbreviation; and the term "paint" and its grammatical variations includes all similar types of coatings that may be described by more specialized names such as "lacquer", "varnish", "primer coat", "top coat", "enamel", or the like.